This invention relates to the field of heat exchangers, and particularly to such exchangers used as coolers in refrigeration systems. It is well known that in use, these coolers or evaporators become coated with ice formed by condensation of the moisture in the ambient air. Such ice acts as a thermal insulator to greatly reduce the efficiency of the refrigeration system. The process of removing this ice is known as defrosting, and can be done manually or automatically, the resulting water in either case being disposed of by known procedures. Automatic defrosting is accomplished by periodically interrupting the flow of refrigerant to the heat exchanger and supplying heat thereto instead, ordinarily electrically, for intervals only long enough to melt off the ice, without materially raising the temperature of the refrigerated space. Various methods of incorporating electric heaters into heat exchangers for this purpose are known, but must be practiced at the time the exchanger is being manufactured and are ill adapted to repair and replacement procedures. This latter point is of great importance, particularly in commercial installations using exchangers of large size permanently installed in locations usually of restricted access.